Federal employees in Montgomery County at agencies such as the National Institute of Standards and Technology returned to work Thursday with the news they would receive back pay for furlough days during the 16-day government shutdown.

Private contractors did not receive the same assurances.

The last time the government shut down, in 1996, contractors were not reimbursed, while federal employees were. When Bethesda defense giant Lockheed Martin furloughed about 2,400 employees a couple of weeks ago because of the political standoff, CEO Marillyn A. Hewson directed them to use vacation time to continue receiving pay.

A Lockheed spokesman could not be reached Thursday to comment on furloughed employees’ potential back pay. Company officials released a statement saying they were pleased an agreement to end the shutdown was reached, and the 2,400 furloughed employees would be “back to work soon.”

“We expect all U.S. government facilities to open, stop work orders to be lifted and for our operations to return to normal as soon as our customers are all back in place and have informed us that we may resume many critical programs that were halted during the shutdown,” Lockheed officials said on Thursday.

Some other area contractors were not affected much by the shutdown. Bethesda enriched uranium supplier USEC initially thought it might have to furlough some employees or slow down work at an Ohio uranium enrichment project if the shutdown ran past Oct. 15.

But that was averted and there was not a significant impact, Paul Jacobson, a spokesman for USEC, said Thursday.

“We had to carefully manage the fiscal 2013 funds that were carried over,” he said. “That allowed us to keep the program operating without pause.”

USEC is building a $350 million plant to produce low-enriched uranium to make nuclear fuel. The project is about 80 percent funded by the U.S. Department of Energy.

The U.S. Senate agreement that was reached late Wednesday to end the shutdown and raise the debt ceiling offered back compensation to federal workers who have been furloughed since Oct. 1.

Many of the 3,600 workers who were furloughed since Oct. 10 at the Rockville-based U.S. Nuclear Regulatory Commission had returned to work Thursday. The agency is rescheduling public meetings, updating reports of incidents at reactors and resuming normal operations, said NRC spokesman Scott Burnell.

About 3,000 of the agency’s 3,900 employees work at the headquarters off Rockville Pike.

As for contractors with the agency, the NRC is reviewing projects to decide which ones will resume first, Burnell said.

“Some contractors continued working during the shutdown, while others did not,” he said.

Gaithersburg-based NIST resumed normal operations Thursday. Contractors who work on the NIST campus were instructed to report to work. A spokeswoman could not be reached for comment Thursday.

In a memo to employees at the Silver Spring-based Food and Drug Administration, U.S. Health and Human Services Secretary Kathleen Sebelius wrote that the decision on which employees to furlough was “based on a technical statutory definition mostly determined by how [an employee’s] work is funded, and not the value” the FDA attributes to workers.

The Bethesda-based National Institutes of Health — which had to delay its grant review process, medical studies and accepting additional patients during the shutdown — furloughed about 70 percent of its almost 19,000 employees.

Each day of the shutdown could cost Montgomery County $760,000 in lost income tax revenue, according to a report by the county’s Finance Department.